1. Field of the Invention
The present invention relates to measurement of loop length in fiber-optic circuits such as sensors and signal processors, and more particularly to a method for the accurate determination of differential propagation delays in such fiber-optic circuits.
2. The Prior Art
Fiber-optic coils are commonly used today as delay lines in various applications. For example, fiber-optic delay lines are utilized in interferometric fiber-optic sensors, fiber-optic signal processors, and fiber-optic lattice filters. The delay lines may be used in combination in interferometers to produce large differences in signal propagation delays, or they may be used in recirculating delay lines to give a large value to the round trip delay.
There is often a need for an accurate determination of the relative propagation delays in optical circuits such as those mentioned above. For example, matching of the path imbalances in pairs of interferometers used in sensor arrays is important to insure that phase-induced intensity noise does not arise to a measurable level on the signal pulse propagating within the interferometers. Without matching of the paths, the uncompensated imbalances in the various loop lengths require the use of long coherence lengths in signal sources, and/or the uncompensated imbalances introduced phase-induced intensity noise into the signals propagating within the sensor arrays.
The performance of fiber-optic lattice filters depends on the tolerances of the filter delay line lengths. These tolerances may reach 1 millimeter in 100 meters or, in other words, 10 parts per million (PPM).
Various techniques have been applied in the past in efforts to measure propagation delays in fiber-optic circuits and, in particular, to measure the differential propagation delays in pairs of optical circuits used in applications such as those described above. These techniques have been subject to limitations which have made it difficult to achieve the tolerances necessary for optimum design.
For example, one technique which has been used for somewhat different applications is referred to as optical time domain reflectometry. This technique has often found application in monitoring the integrity of optical fiber communications systems and other long line optical systems. The technique comprises launching a narrow pulse of light into an optical fiber, wherein it is continuously Rayleigh scattered as it propagates along the fiber. Some of the scattered light will be returned to the launch point. The intensity information present in this back scattered light is used to determine the distribution of attenuation along the optical fiber. This attenuation information is particularly useful for locating bad joints and anomalously lossy sections of fiber. It has been proposed to also use polarization information contained in the back scattered light, to determine the distribution, along the length of the fiber, of external influences which effect the polarization state of the light propagating in the fiber.
Such a system is useful in locating breaks, bad joints and the like or in detecting environmental conditions in long optical fiber systems such as communication systems which have a break somewhere along a 1 kilometer length of line. This system can be used to approximately identify the location of concern, within a tolerance which is probably on the order of 10 centimeters. Although such a technique finds application as described above, it cannot provide measurements which even approach the tolerances necessary to find useful application in the determination of propagation delay in sensing, signal processing and filtering circuits.
Another technique which has been utilized for determination of differential signal propagation time in optical fiber sensors and similar circuits is a technique referred to as optical frequency domain reflectometry. In this technique, an optical input signal is frequency modulated by use of a ramp frequency in time. The frequency modulated signal is communicated into a pair of optical waveguides having different optical path lengths. The signals propagated from the pair of waveguides are mixed and the mixed signal is monitored to determine the difference in frequency at each moment. This difference provides a measure of the difference in optical path length of the pair of waveguides.
This optical frequency domain reflectometry technique is able to meet many of the required tolerances of optical circuits such as sensors and signal processors. However, because frequency modulation is utilized, the technique requires that the path length difference which is being measured be shorter than the coherence length of the laser. As a result, such a system can only measure moderate differences in arm lengths. The maximum difference is, of course, dependent upon the coherence length of the source. Based upon optical sources which are commonly available in the commercial marketplace, such a difference is typically not more than a range of about 2-10 meters, depending upon the optical source. In addition, this technique is limited with respect to the minimum difference which can be detected to an amount on the order of about 15 centimeters. Thus, because of the complexity of the equipment required for accomplishing such techniques, as well as the limited ranges in which the technique can be applied, there are many applications which cannot be serviced by such techniques.
For example, one technique which may be used for measuring the differential propagation delays comprises an impulse response measurement of the system. Such a measurement consists of putting a short optical pulse into the system and detecting the resulting pulses at the output of the fiber-optic system. A measurement may be made of the time delay between adjacent pulses to provide a measurement of the optical path length difference between adjacent arms in the fiber-optic system. This method suffers from several disadvantages, including a limitation on the accuracy with which such measurements may be made, depending upon the width of the input pulse from the source. In addition, the cost of equipment associated with this type of method rises rapidly with the requirement for improved resolution. This results from the fact that both lasers and detectors necessary for detecting short pulses become increasingly expensive as the width of the pulse which is to be produced or detected in decreased.
In view of the above, it would be a great improvement in the technology to provide a reliable and easily implemented technique to accurately measure the difference in propagation delays in fiber-optic circuits. It would be a further improvement in the technology to provide such a technique and system which could provide differential propagation length measurements at very small tolerance values and throughout a very broad range of optical path length differences. It would be a still further improvement in the technology if such a technique were provided which made use of the properties and characteristics of the circuits themselves when making the measurements.